The invention relates to an interferometer comprising a beam splitter for splitting an incident radiation beam into two sub-beams following first and second separate paths, first and second fixed mirrors terminating the first and second paths and serving to reflect the sub-beams back to the beam splitter, the beams splitter being further arranged to re-combine the sub-beams, at least one of the first and second paths including path length variation means.
Such a system is known from EP 171 837 in which a beam splitter, two fixed mirrors and a rotatable mirror set define the basic structure of the interferometer. In this interferometer a variation in optical path length can be realised relatively simply and the construction of the optical arrangement is relatively straightforward.
A disadvantage of this solution is that the optical elements listed need to be mounted in accurately defined positions relative to each other and these positions are relatively widely spaced and in different orientations.
It is an object of the present invention to mitigate this disadvantage.